The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for rapid MRI of vascular calcifications and other magnetic susceptibility-shifted substances.
Vascular calcifications are a major component of atherosclerotic disease, and their presence is a risk factor for future cardiovascular morbidity and mortality. In clinical practice, computed tomography (CT) is used to visualize and quantify vascular calcifications. CT, however, involves the use of potentially harmful ionizing radiation and thus is not well-suited for longitudinal and/or serial assessments of atherosclerotic disease, or for screening in the general population.
MRI is a promising imaging modality for visualizing and quantifying vascular calcifications because it does not use ionizing radiation and thus does not have the safety concerns of CT. However, vascular calcifications generally appear dark in magnetic resonance images, and are often confounded by image artifacts, such as chemical-shift artifacts that appear at fat-water interfaces
Chemical-shift artifacts are typically reduced by the application of frequency-selective radio frequency (RF) pulses tuned to the Larmor frequency of lipid spins. However, for the purpose of imaging vascular calcifications, it is important to maintain the bright signal from fat so as to contrast with the low signal from the vascular calcifications. Dixon-type techniques can eliminate chemical-shift artifacts by creating separate fat-only and water-only images; however, it is necessary to depict both water and fat together in a single image in order to provide optimal contrast with vascular calcifications.
Other approaches for imaging vascular calcifications that have been proposed fail to rapidly and accurately identify calcifications. So-called “multi-contrast” MRI, which includes acquiring a dark-blood T1-weighted fast spin echo (FSE) scan, a dark-blood T2-weighted FSE scan, a dark-blood spin-density weighted FSE scan, and a bright-blood time-of-flight (TOF) scan, is one such approach. This multi-contrast approach identifies vascular calcifications based on their dark appearance in all four acquisitions; however, the method requires long scan times associated with acquiring data using multiple (typically four) different scans, and thus is not a rapid technique. Accuracy in the multi-contrast technique also suffers due to poor spatial resolution (e.g., because the FSE techniques are typically 2D and not 3D techniques) and to arterial inhomogeneity and saturation artifacts in TOF imaging. Furthermore, interpretation of multiple image sets is complex and can be impossible due to patient motion in one or more scans.
Another approach for visualizing calcifications involves the use of 3D dark-blood acquisitions (either FSE or gradient-echo) that display the arterial wall. This approach does not allow for rapid and accurate identification because calcifications are visualized based on their dark appearance, but can be hard to distinguish from the many other structures within the imaging field that appear dark, including vascular lumen and perivascular fat. An offshoot technique referred to as “gray-blood imaging” addresses the poor contrast between superficial vascular calcifications and the vascular lumen, but does not solve the issue of poor contrast between perivascular fat and vascular calcifications.
Ultrashort echo time (UTE) MRI could potentially be used to visualize vascular calcifications by the subtracting the signal intensity of a long echo time (e.g., TE typically on the order of a few milliseconds) from that of an ultrashort echo time (e.g., TE significantly less than one millisecond). However, UTE methods are hindered by artifacts from gradient timing errors and poor spatial resolution. Experience with these methods has largely been limited to in-vitro imaging and it is unclear if these methods would work for in-vivo imaging of vascular calcifications despite the drawbacks noted above.
Two other methods for visualizing calcifications with MRI have recently been used. The first method includes using a non-selective 3D UTE sequence. Drawbacks of this method include long acquisition times and sensitivity to artifacts from physiologic motion (e.g., from breathing). The second method is a 3D Cartesian acquisition that uses an in-phase echo time. Drawbacks of this method include chemical shift artifacts in the frequency-encoding direction and sensitivity to physiologic motion.
Thus, there remains a need for MRI techniques that are capable of rapidly and accurately imaging vascular calcifications.